The present invention relates to the generation of pulsed supersonic gas flow from a slit-shaped (i.e., long and thin) orifice.
Supersonic jets produce cold, gas-phase molecules which move along well defined streamlines with a narrow velocity distribution. These gas-phase molecules are usually generated by expanding a warm, high pressure gas across an orifice into a vacuum chamber.
Some of the more important applications of supersonic jets include isotope enrichment, wind-tunnel experiments, and molecular beam studies. See, for example, Feen et al., "Free-Jet Experiments in a Spacecraft Environment", Final Report on Contract #954327 between Calif. Institute of Technology Jet Propulsion Laboratory and Relay Development Corporation.
Especially important as both an application and a diagnostic tool for supersonic jets has been the field of molecular spectroscopy, in which the interaction between the molecules in the jet with one or more beams of light is studied. (See, for example, Levy, Ann. Rev. Phys. Chem., 31: 197 (1980); Levy, Scientific American, Feb., 1984, p. 96; Vaida, Accts. Chem. Res., 19, 114 (1986)).
The use of supersonic jets of rare gases seeded with large molecules provides a source of internally cold, isolated large molecules which can be probed by a variety of spectroscopic techniques; see, for example, Amirav et al., Anal. Chem., 54: 1666 (1982).
One of the more popular techniques has been laser-induced fluorescence (LIF). In LIF, the high intensity of a laser beam compensates for the relatively low molecular density in the jet, making excitation of molecules easy, while fluorescence detection provides high sensitivity. Since supersonic jets eliminate vibrational sequence congestion as well as rotational congestion in the spectra of polyatomic molecules, linewidths of spectral features are often reduced by factors of 1000 or more, compared to conventional spectroscopy, making high-resolution studies of molecular energetics and dynamics possible. For examples, see Fitch, et al., J. Chem. Phys., 70: 2019 (1979); Amirav, et al., J. Chem. Phys., 71: 2319 (1979); Hopkins, et al., J. Chem. Phys., 72: 5039 (1980); Oikawa, et al., J. Phys. Chem., 88: 5180 (1984); Felker and Zewail, J. Chem. Phys., 82: 2994 (1985).
While many of the jets described in the literature have circular orifices, an important variation on this apparatus involves the use of a slit-shaped orifice. Slit-shaped orifices produce supersonic jets having important differences in gas properties, compared to jets from circular orifices; these properties have been characterized theoretically and experimentally, for example, in Sulkes et al., Chem. Phys. Lett., 87: 515 (1982); Beylich, in Paper No. 111 and Dupeyrat, in Paper No. 135, delivered at the Twelfth International Symposium on Rarefield Gas Dynamics, Charlottesville, Va., July 7-11, 1980.
One of the major advantages of a slit-shaped orifice over a circular one is that, for a given total gas flow rate, the slit provides a much greater interaction length with a light beam crossing the jet at right angles. For most types of spectroscopy, this results in important gains in sensitivity. For direct absorption measurements, in particular, slit nozzles have made some experiments feasible for the first time. See, for example, Amirav and Jortner, J. Chem. Phys., 82: 4378 (1985).
Regardless of the shape of the orifice, supersonic jets require high pressure ratios across the orifice, which are normally achieved by maintaining a vacuum on the downstream side of the orifice. Since the flow rate through the orifice is large, the vacuum pumps required to handle the flow of a continuously operating jet can be large and expensive. In many applications, it is acceptable or even desirable to operate the jet intermittently (see, for example, Zwier, et al., J. Chem. Phys., 78: 5493 (1983)), reducing the size of the vacuum pumps required. Thus, the ability to rapidly switch the jet on and off can have great practical importance.
Circular orifices can be rapidly opened and closed using valves of various designs, several available commercially. In contrast, fast, effective valves for slit-shaped orifices are uncommon. Only one has been described in detail in the literature; and this device is addressed herein below in order to demonstrate both its utility and its shortcomings, as well as to compare it to the present invention.
Amirav, et al., Chem. Phys. Lett., 83: 1 (1981), described a pulsed slit nozzle for the production of pulsed, planar supersonic jets. The source was constructed from two concentric cylinders, with matching slits of dimensions 0.2 mm wide and 35 mm long machined in each cylinder, parallel to the cylinder axis. The internal cylinder (70 mm long, diameter 20 mm, wall thickness 0.5 mm) was spun by a motor. The external cylinder had an inside diameter which matched the outside diameter of the internal cylinder with a tolerance of 0.02 mm. MoS.sub.2 powder was used as a lubricant between the cylinders.
The pulsed, supersonic nozzle slit source had a repetition rate of 12 Hz and a pulse wdith of 150 microseconds. The source could be heated up to about 200.degree. C. A sample of the molecules was placed near the inner cylinder, heated to give a vapor pressure of about 0.1 Torr, and mixed with Ar gas in the pressure range of about 20 to 100 Torr, which was fed into the inner cylinder.
The pumping system consisted of two mechanical pumps, with the pumping speed of the system being about 700 liter/min. Light from a tunable pulsed dye laser crossed the supersonic gas expansion parallel to the long axis of the slit at a distance ranging from 6 to 15 mm from the source. The temporal coincidence between the laser pulses and the supersonic gas pulses was achieved by use of an IR optical switch and a variable delay unit.
In typical applications, the authors performed absorption and/or LIF studies on medium to large-sized organic molecules. See, for example, Amirav, et al., Chem. Phys., 67: 1 (1982); Bersohn, et al., J. Chem. Phys., 79: 2163 (1983); Sonnenschein et al., J. Phys. Chem., 88: 4214 (1984).
This design represented the first successful operation of a pulsed nozzle using a long, thin orifice.
However, theoretical considerations, as well as our own experience with a nozzle built to similar specifications, point out a number of problems with the design.
For example, gas leakage when the nozzle was closed (the static leak) was a serious problem. To some extent, this is an inherent problem in all pulsed slit nozzles: at the very least, the nozzle must seal around its entire perimeter; for a given orifice area, this problem is far worse than in the case of a circular orifice, which, in fact, has a minimum perimeter: area ratio. The greater the aspect ratio (ratio of length to width) of the slit, the farther from this minimum is the actual ratio, and slit nozzles are usually employed in applications where very large aspect ratios are desirable.
However, in the Amirav design, a seal must be maintained around the entire surface area of the inner cylinder (i.e., its cylindrical surface and both ends); this is much greater than the minimum sealing problem, involving as it does the regions where bearings must be located and gas supply line and motor shaft must be connected. The authors noted that their device operated with a 0.02 mm gap between the two cylinders; this can represent a substantial leak at high gas pressures, and the situation is likely to be exacerbated with wear.
Another potential problem is the use of a motor to rotate the inner cylinder. If the motor is located inside the vacuum chamber, it must be equipped with a pressurized housing containing air or some other gas to prevent burnout; if it is outside the vacuum chamber, a rotary-motion feedthrough is required. These alternatives add complexity, cost, and/or potential leaks to the system.
Finally, as far as can be determined from published accounts, the Amirav et al. pulsed slit was operated at a more or less constant angular velocity. Thus, regardless of the actual value of this velocity, the gas was on for a fixed fraction of the time (defined as the duty cycle), determined by the width of the slits and the diameter of the matched cylindrical surfaces. The only way to change the duty cycle (for example, if one desired to use higher backing pressure without a concomitant rise in the pressure in the vacuum chamber) would be to remachine the entire device.
Furthermore, external control of exactly when the nozzle is open is very difficult to achieve with an electric motor drive, as it requires a means for fine control of acceleration and/or deceleration to ensure that the slits cross at exactly the desired time.
The present invention addresses and mitigates the problems of this prior art device.